The two-stroke engine is often preferred over the four-stroke engine in the field of general purpose engines because of the simplicity in the structure. The two-stroke engine is often not provided with any lubrication system, and the required lubrication is achieved by the use of mixed fuel containing lubricating oil. As the fuel is supplied to the crank chamber before being forwarded to the combustion chamber, the various rotating parts and sliding surfaces are lubricated. The lubricating oil is also known to be useful in removing heat from various moving parts. However, because the lubricating oil supplied in this manner is highly limited in quantity, as compared to the case of a four-stroke engine equipped with a lubricating oil recirculation system, cooling of the piston and the cylinder is a major problem in the two-stroke engine.
It has been proposed to form a passage in a two-stroke engine opening out toward the back side of the piston, and to blow compressed air against the back side of the piston via this passage. See JP04-237815A, for instance. It has also been proposed to provide a guide plate extending at an angle to the cylinder axial line on the back side of the piston such that gas may be guided to the back side of the piston as the piston moves downward for the purpose of cooling the piston. See JP61-101655A, for instance. In the two-stroke engine disclosed in JP61-101655A, an opening is formed in the skirt of the piston so as to communicate with the intake port during the downward stroke of the piston. The mixture mixed with fuel in the intake port is conducted through the intake port, the opening, a space on the back side of the piston and the crank chamber, in that order, and this flow cools the piston. According to yet another proposal, an opening is formed in the skirt of the piston such that the mixture may be conducted through the crank chamber, a space on the back side of the piston, a scavenging port and the combustion chamber. See JP03-026820A, for instance.
However, according to the invention disclosed in JP04-237815A, in addition to a pump, special passages are required in the engine so that the complexity and the size of the engine inevitably increase. As for the invention disclosed in JP61-101655A and JP03-026820A, the piston is required to have a skirt having an opening formed therein so that the compatibility is impaired, and the manufacturing cost increases.